In order to meet higher capacity demands and enhanced user experiences, cellular communications networks such as Long Term Evolution, LTE, need to be deployed with an increasing density of base stations. This densification can be achieved by splitting macro cells and by deploying small cells in highly loaded geographical areas, or so called traffic hotspots, within the coverage area of macro cells. With densification of cellular networks, radio resources can be further reused and users generally will be closer to their serving base stations, which enables higher bitrates.
Cellular networks with a mixture of macro cells and small cells with overlapping coverage areas are sometimes referred to as heterogeneous networks. These types of networks are seen as an important complement to macro cell splitting. One example of such deployments is where clusters of pico cells are deployed within a macro coverage area to offload macro traffic. A pico base station represents one example of a low power node, LPN, transmitting with low output power and correspondingly serving a much smaller geographical area than a high power node, such as the typical macro base station. Other examples of low power nodes are home base stations and certain types of relays.
A consequence of network densification is that wireless devices, such as user equipments (UEs), operating in the network will experience lower geometries, which implies that downlink inter-cell interference can be more pronounced and thus limit the achievable bit rates. Hence, in dense cellular deployments, interference mitigation techniques have the potential to substantially improve the user performance. Interference mitigation can either take place on the transmitter side or on the receiver side, or a combination of both. Interference mitigation techniques often exploit the structure of the physical layer transmission used in the involved radio access technology.
Regarding receiver-side of techniques for mitigating inter-cell interference, interference rejection combining or IRC is a well-known receiver type for suppressing interference. IRC processing requires estimation of an interference/noise covariance matrix. Such matrices express the covariance of interference between the signals being combined via IRC processing. More advanced receiver types for interference mitigation are based on interference cancelation or IC. With IC processing, unwanted received signals, e.g., intra/inter-cell interference, are estimated and subtracted from the “overall” received signal. In this regard, the overall received signal can be understood as being a composite of desired and undesired signals impinging on the receiver antennas.
Maximum Likelihood, ML, is another interference mitigation technique. ML-type receivers recover transmitted symbol information from a received signal based on jointly detecting symbol information from multiple signals, e.g., from several different cells. The joint decision is determined based on minimizing a joint error metric. ML-type receivers usually rely on searching among all possible combinations of defined symbol values, which are also referred to as modulation constellation points.
IRC and IC were established as UE reference receiver techniques in Release 11 of the applicable LTE technical specifications by the Third Generation Partnership Project, 3GPP. However, in LTE Release 11, also referred to as Rel-11, IC was restricted to cancelation of always-on signals, and the network assisted the UE as to how such signals were transmitted in the aggressor cells. The Common Reference Symbols or CRS, which are transmitted in LTE networks on a cell-specific basis, represent one type of always-on signal for which IC-based interference mitigation would be performed.
However, there is a growing interest in developing approaches for the cancelation and suppression of interference corresponding to scheduling of data, and such features are an item of interest in ongoing work for LTE Rel-12. Consequently, IC receivers in UEs for mitigating downlink interference arising from neighbor-cell data transmissions are now gaining popularity. The IC receiver in the victim UE—i.e., the UE experiencing the interference at issue—demodulates and optionally decodes the interfering signals, and produces an estimate of the transmitted and the corresponding received interfering signal. The receiver then removes that estimate from the overall or total received signal, to improve the effective signal-to-noise-plus-interference ratio, SINR, for the desired signal.
In post-decoding IC receivers, the interfering data signal is demodulated and decoded. The decoding results and channel estimates for the interfering signal are used to estimate the interfering signal's contribution to the composite received signal—i.e., interfering signal as received by the IC receiver is regenerated from the decoding results. The regenerated signal is then removed from the composite received signal, for improved demodulation and decoding of the desired signal or signals from the composite received signal. Post-decoding IC receivers are sometimes referred to as Code-Word IC, CWIC, receivers.
As an alternative to post-decoding IC processing, pre-decoding IC receivers perform the regeneration step directly after demodulation, thus bypassing the channel decoder with respect to the interfering signal at issue. That is, a pre-decoding IC receiver performs symbol detection with respect to an interfering signal but does not provide the detection results to its decoder. Instead, the detection results are used, e.g., in a “soft” symbol mapping process, to regenerate the interfering signal, for removal from the composite received signal. Pre-decoding receivers are sometimes referred to as Symbol Level IC, SLIC, receivers.
The term cancelation efficiency or CE of an IC receiver denotes the fraction of impairment (interference plus noise) power remaining in the received signal, after the receiver performs cancelation processing. The CE for the pre- and post-decoding IC approaches may be essentially equal in some scenarios and vary significantly in others. For example, the post-decoding IC approach typically provides superior performance at “high” SINR operating points. The preferred approach is based on applying soft signal mapping and regeneration, as opposed to using hard symbol or bit decisions.
In many IC receiver architectures, as well as IRC and ML architectures, some prior knowledge about the interfering signal is required to perform interference mitigation or to enhance the performance of such mitigation. Basic information includes knowledge about at least a subset of the resource allocation of the interfering signal, modulation-related parameters of the interfering signal, and coding-related parameters of the interfering signal. For example, in LTE, resource allocation knowledge would mean knowing at least some of the Resource Blocks or RBs used for the interfering signal. In networks that use High-Speed Packet Access, HSPA, the resources in question are codes used on the High-Speed Physical Downlink Shared Channel or HS-PDSCH. Example modulation-related parameters include transmission mode, modulation format, Multiple-Input-Multiple-Output, MIMO, rank, precoding weights, etc. Example coding-related parameters include transport block size, code rate, etc.
Receiving a neighbor-cell downlink, DL, control channel represents one mechanism for obtaining knowledge about interfering data transmissions in the neighbor cell. More particularly, in advance of a neighbor-cell making a downlink data transmission to a given neighbor-cell UE, a control message is sent on the control channel of the neighbor cell. That control message carries resource allocation information, transport format information, etc., for use by the targeted neighbor-cell UE in receiving the upcoming data transmission.
In LTE, such a control channel is referred to as the Physical Downlink Control Channel or PDCCH, while HSPA-based networks use a High Speed Shared Control Channel or HS-SCCH. Although the neighbor-cell control channel may be power-controlled towards the intended neighbor-cell UE and not toward the victim UE, in many scenarios of interest, the victim UE nonetheless experiences signal quality sufficient for decoding the control message associated with the interfering signal. From this point forward, the term “IC receiver” refers to a receiver that can mitigate neighboring cell interference. Examples of such receivers are IRC, ML, SLIC and CWIC.
In LTE, HSPA and other contemporary cellular networks, the DL transmissions to UEs use fast link adaptation. In this scheme, a given UE signals to its serving cell the channel quality experienced by the UE at the current scheduling interval, which for LTE is the current subframe and in HSPA is the current Transmission Time Interval or TTI. Here, it will be understood that the LTE subframe is functionally the same as the HSPA TTI, in the sense that they both represent a basic scheduling interval. The UE further indicates the preferred rank and precoding properties. The serving radio node (or base station or NodeB) receives this information and schedules a DL transmission several subframes or TTIs later using a transport format that the UE is able to successfully decode, assuming the previously reported channel quality. Problematically, however, the interference properties at the current and future subframes or TTIs are different, and the achievable IC gain at the UE for the future subframe or TTI is difficult to predict.
The difference between reception conditions as they exist at the UE when the scheduled transmission is later received and as they existed at the earlier reporting time often means that the CE of IC processing at the UE during reception of the scheduled transmission does not match the CE achievable at the time channel quality was reported. Fortunately, in practical networks, the serving radio node usually applies some type of adjustment to reported channel quality to obtain a desired long-term target Block Error Rate or BLER. This leads to more aggressive transport format, TF, scheduling for IC-capable UEs, as compared to non-IC UEs with linear receivers, and it helps in the realization of average throughput, TP, gains and cell capacity gains from IC.
However, there are existing approaches directed to the mismatch between channel quality as reported by a UE versus actual channel quality during a later transmission to the UE. For example, an IC UE may predict the transport format, TF, that will be used by a neighbor-cell base station when making a transmission that will interfere with the UE's reception of scheduled data. The UE then adjusts its current channel quality report in view of the predicted TF. That adjustment reflects the sensitivity of CE to the TF of interfering signals. Of course, the adjustment is imperfect in the sense that the prediction may be wrong.
In another approach, an IC UE reports own-cell channel quality and additionally reports channel quality with respect to one or more interfering neighbor cells. The serving radio node uses the additional information to better estimate what the channel quality will be at the UE during the later-scheduled transmission to the UE. Problematically, however, these approaches do not address the problem recognized herein. Namely, by not accounting for the actual TF of the interfering signals at the UE at the actual time of transmission to the UE, the network fails to realize the throughput and performance gains possible with IC UEs, by using link adaptations that are either too aggressive or not aggressive enough, which leads to reduced system capacity as a consequence of not fully exploiting the channel capacity.
For example, overly aggressive TF selection causes an increased incidence in retransmissions and hence uses more time-frequency resources than necessary. On the other hand, if TF selection is too conservative with respect to actual channel conditions at the UE during the transmission interval at issue, the UE is served at an effective SINR that is above what is needed for reliable decoding of the transmitted data.